REF Table or Attribute

This depends on what you are trying to accomplish. If you are simply wanting to record the target's age, a person can only have one age so that one to one relationship would make it an attribute in your existing table.

However, if you want to have pre-determined bands of ages to make demographics you are creating a dimension as a ref table. The join conditions may be tricky here for the below reason.

The thing I'm most concerned about here is maintaining this data in a controlled way. Age is something that changes and people will change bands as they get older which requires a maintenance script or ETL process to run regularly to make sure every target is accurate.

For this reason I would recommend that you add a Date_Of_Birth attribute to your existing table. You can then create a view that does some date logic on the DOB field to make an age range column.